There is an inner flow in liquid films that may appear calm and motionless on the surface. For the first time, European scientists are looking into the hidden shapes of liquids evaporating in space.

This image was captured using a technique called interferometry, which makes the vapour above the liquid visible in a way the naked eye never could. Each wavy band works like a contour line on a map, tracing points where the vapour has the same concentration instead of the same height. Read together, the bands turn an invisible cloud into a picture you can follow, above a liquid film only a fraction of a millimetre thick.

These patterns, and the flow patterns in the evaporating liquid itself, are shaped by surface tension. This force-driven flow in what physicists call the Marangoni effect. The phenomenon was first noticed by James Thomson in the 19th century while studying "tears of wine" clinging to a glass.

On Earth, gravity tends to flatten both the liquid and vapour above it, obscuring the subtle mechanisms developing at the fluid’s surface. Scientists have now brought a volatile and non-flammable liquid to the International Space Station to investigate the phenomenon in microgravity.

ESA’s Marangoni in Films experiment studies how evaporating liquid films behave in space and how their motion can be controlled. Observing both the liquid and the vapour at the same time allows researchers to study how liquid films organise themselves and transport heat and mass.

Understanding these complex internal flows matters on Earth too, for industries that use liquid films to transfer heat. Applications include cooling technologies, coating and printing processes, distillation and chemical engineering.

More science runs will continue over the summer inside the Heat Transfer Host 2 facility. If you are curious about fluids science happening inside the European Columbus laboratory, check out how condensation defies gravity.